This invention relates to improvements in devices for pressure fixing electroscopic toner powders at ambient temperatures and more particularly to the technique of mounting such rollers under conditions that would minimize the flexural stresses and deflection introduced into such rollers under heavy load conditions.
Electrostatic copying methods and apparatus for accomplishing reproduction of graphic subject matter are well known in the art. One of the important steps in such a reproduction process is the fixing of the powder image on the surface which is ultimately to become the copy. The electroscopic powders or toners, as they are known, are generally thermoplastic in nature and heat responsive. It is conventional in this art to use heat to fuse such thermoplastic materials to the copy sheet. The combination of heat and pressure has also been used to fix the powder to the copy sheet.
The use of pressure, without any heat whatsoever, has been suggested whereby the toners may be fixed to the copy sheet. Such a method and apparatus are fully described in U.S. Pat. No. 3,854,975 issued to Richard S. Brenneman, David W. Lovering and Frederick E. Barr granted on Dec. 17, 1974 and assigned to the same assignee of the present application. The term fixing, as it is used in this art, refers to the technique of permanently affixing a powder image to a substrate so that it will adhere and retain its imagewise configuration without disruption as a result of normal handling which is experienced by a copy sheet in normal business usage.
A significant and major advancement in this technology has been made where the advent of the pressure fixing apparatus and method disclosed in the Brenneman, et al., patent, in overcoming the disadvantages of using heat systems to cause the powder image to adhere to the base support or copy sheet. The use of such an invention in which the electroscopic powder is fixed by pressure alone, that is, without heat, is not without disadvantages. The conditions of applying high compressive forces on such a device impose stringent requirements on its design, and in particular the alignment of rollers with respect to one another.
One solution to such a problem is to slightly skew the rollers, which is a technique well known in the art and conventional in overcoming the problem of non-uniform pressure across the operating length of such rollers. While intentional skewing of the rollers solves the problem of nonuniform pressure, it introduces other problems such as wrinkling sheets of paper that pass through the system. The problem of wrinkling sheets of paper which pass between skewed rollers becomes aggravated when the sheets of paper are used in a high humidity environment.
The primary problem in using rollers to which is applied a very high compressive force at the bearing mounts is the bowing of deflection that occurs in the shaft thereby imparting a camber to the roller. The effect is to have the rollers under such high compressive forces move away from one another at the center by a distance that is directly related to the increased compressive force applied at the ends of the roller. This produces high pressure at the edges but lower pressure at the center. This uneven pressure results in an uneven fixing of the developed image.
Skewing the roller will tend to minimize the problem, but it is desirable to have a minimum amount of angular skew of the roller so as to minimize the wrinkling problem. It will be appreciated that as the length of the rollers are extended in order to accommodate wider sheets of paper, the pressure is decreased at the nip of the rollers for a given compressive force. In order to effectively fix the powder image, it then becomes necessary to increase the force for longer rollers. This results in greater deflection. In order to combat greater deflection, wider diameter rollers can be used. It will readily be appreciated that such a piece of equipment becomes cumbersome, unwieldly and costly to manufacture.
The present invention is directed to reducing the amount of skewing or offsetting of the rollers and thereby minimizing the problem of wrinkling but also to substantially decrease the amount of deflection for a given load while keeping the size and the construction of the equipment within reasonable limits. The invention is directed to a roller construction in which the ends of the shaft or core of the roller are fitted with an elastomeric collar on which is supported a tubular casing so as to form an annular space between the shaft and the casing.